1. Field of the Invention
The present invention relates to an optical recording medium on/from which information is recorded or reproduced, and more particularly to a method of manufacturing an optical recording medium which requires an annealing treatment between information tracks.
2. Related Background Art
As a rewritable high-density recording system in a magnetooptical recording medium, there is a system in which a magnetic domain is written in a magnetic thin film by means of the thermal energy of a semiconductor laser to record information, and the information is read by using a magnetooptical effect. In recent years, a demand has increased that the recording density of the magnetooptical recording medium using that system be further increased to realize the large-capacity recording medium.
The linear recording density of the magnetooptical disk serving as a magnetooptical recording medium greatly depends on the laser wavelength of a reproducing optical system and the numerical aperture number of an objective lens. However, an improvement in the laser wavelength of the reproducing optical system and the numerical aperture number of the objective lens is limited. For that reason, there has been developed a technique of improving the recording density by devising the structure of the recording medium or a reading method.
For example, there is a technique disclosed in Japanese Patent Application Laid-Open No. H06-290496. According to this publication, in a structure of a multilayer film having a magnetic domain wall moving layer and a record holding layer which are magnetically coupled with each other, the information is recorded in the record holding layer. At the time of reproducing the information, the magnetic domain of a recording mark in the magnetic domain moving layer is moved by using a temperature gradient which is attributable to the irradiation of an optical beam without changing the information that has been recorded in the record holding layer. Then, the magnetic domain wall moving layer is magnetized so that a partial region of an optical beam spot has the same magnetization, and a change in an optical beam reflected light on a polarization plane is detected, thereby reproducing the recording mark of a light diffraction limit or lower.
This method makes it possible to realize a magnetooptical recording medium and a method of reproducing the magnetooptical recording medium which can reproduce the recording mark of the light diffraction limit or lower and remarkably improve the recording density and the transfer speed.
In the magnetooptical recording medium, in order to facilitate the movement of the magnetic domain of the recording mark in the magnetic domain moving layer by using the temperature gradient which is attributable to the irradiation of an optical beam, a laser beam of a high power is irradiated onto both of adjacent grooves between which an information recording and reproducing track is interposed to conduct a high temperature heating treatment on the groove (hereinafter referred to as “annealing treatment”). Thus, the annealing treatment that alters the recording medium layer of the groove portion is conducted. The annealing treatment makes it possible to obtain the effect that the magnetic domain wall which forms the recording mark does not become a closed magnetic domain. As a result, since coupling between the magnetic layers of the adjacent information tracks disappears, the movement of the magnetic domain wall is more stably moved. Through the annealing treatment, the excellent reproducing signal can be obtained.
However, in the above conventional example, in the case of assuming the annealing treatment, because the groove is annealed, there arises a problem in that only a land can be used as the information track, and it is difficult to achieve the high density.
Also, in recent years, to aim for the realization of the higher density, a study related to a magnetooptical disk that can use the land portion as well as the groove portion as the information track which requires no annealing treatment has been popularized. According to this study, the high density can be conducted perpendicularly with respect to the information track of the magnetooptical disk.
For example, in Japanese Patent Application Laid-Open No. H11-195252, there is realized a medium that can use not only the land portion but also the groove portion for recording (hereinafter referred to as “land/groove recording medium”) by controlling the surface roughness of the groove side wall in a substrate.
In the above manner, conventionally, the tracks of the magnetooptical recording medium allow a narrow track pitch of up to about 0.5 μm. According to the experiment, the recording/reproducing of 0.08 μm/bit is realized as the linear recording density at a practical level by using the land/groove substrate having deep grooves (the groove depth is about 160 nm) with a track pitch of 0.54 μm. This corresponds to 15 Gbit/inch2 as the recording density.
Also, in the case where recording can be performed on not only the land but also the groove, a relatively deep groove of about 100 nm or more is assumed. For that reason, temperature distributions that are produced at the time of tracing the land portion and at the time of tracing the groove portion are largely different from each other. In particular, a recording power that is relatively larger than that at the time of tracing the groove portion is required at the time of recording the land portion. Therefore, there arises a problem in that data on the groove portion is destroyed, that is, so-called cross-written when recording on the land portion is conducted with the optimum recording power.
Also, there arises a problem in that molding is difficult and the track pitch is difficult to narrow in the deep-groove substrate.
In view of the above circumstances, as a method of solving the above problems, the applicant (assignee) of the present invention has submitted a proposal that in order to use both of the land portions and the groove portions of the relatively shallow grooves as the tracks with a relative step between the land portions and the groove portions being about ⅛ or less of the light source wavelength related to the recording and reproducing, the lands and the grooves are magnetically divided through a method of altering the physicality of boundary portions (hereinafter referred to as “side walls”) between the lands and the grooves through the annealing treatment of the laser beam irradiation in advance.
According to the above method, it is possible to provide the magnetooptical recording medium that can perform higher density information recording of the narrower track pitches, which corresponds to the land/groove recording using the shallower groove having tracks magnetically divided in the radial direction of the recording medium.
In annealing the side walls between the lands and the grooves, there has been studied a simple and high-efficient method. In view of the above circumstances, as a result of studying the above annealing treatment earnestly, the applicant (assignee) of the present invention has found out the following fact. That is, after a certain side wall is annealed with a laser power for annealing in order to anneal another side wall adjacent to the side wall, the side wall is annealed after traversing a track with the laser power for annealing as it is (called “track jump”). In this case, the region that has been annealed at the time of traversing the track is formed in the information recording track, that is, the land portion or the groove portion. For that reason, the applicant of the present invention has apprehended a problem in that a region where no recording magnetic domain is formed in the information track occurs, reducing an effective recording region.